Shaky Car

Hi all, i have a 99 1.8 zetec mondeo which i have recently bought. When yput your foot down between 45 and 70 the car has a shake on it. I thought it might of been the wheels out of balance. ive ad the wheels balanced but there has been no change. Ive noticed today that the harder and faster i put my foot down the shake is more violent. The shake doesnt come from through the steering wheel, but through the chassis and as if the engine is shaking. any thoughts would be welcomed.
Thanks
Damian